Gases, such as oxygen, nitrogen or argon, are used in a large number of different applications. For example, oxygen is used for metals production, such as stainless steel refining, and by hospitals for life support, and nitrogen and argon are used for inert gas blanketing of chemically reactive processes and for inert gas purging of flammable or explosive furnace atmospheres.
Because of the very significant volume reduction of the liquefied gas, the gases are generally transported in liquid form to the usage site such as the factory or hospital, and stored as liquid at the usage site in a liquid storage tank, As the usage point, i.e., the hospital or factory requires gas, the liquefied gas is passed out of the storage tank, at pressure or pumped if necessary, heated to vaporize the liquid and warm the cold gas to close to ambient temperature, and then passed on to the usage point.
The liquefied gas is heated generally in one of two ways, either by heat exchange with ambient air or by heat exchange with a heated fluid.
The use of an atmospheric vaporizer to vaporize the liquefied gas offers the advantage of reduced energy usage since the heat for liquid to warm gas conversion is attained from the ambient air at no cost. One disadvantage of an atmospheric vaporizer is the comparatively large size that is needed to heat liquid to achieve any given flowrate compared to the size of a powered heat exchanger to achieve the same flowrate. Another disadvantage of an atmospheric vaporizer is unreliability due to the buildup of frost or ice on the atmospheric side of the heat exchange surfaces. The rate of this buildup is affected by a large number of variable and uncontrollable factors such as the ambient temperature, the relative humidity, the wind velocity, the solar exposure and the type and amount of precipitation. Thus, frost or ice buildup may occur in a highly irregular pattern which gives rise to the possibility of unexpected vaporizer fouling and thus the inability to supply product gas to the use point at the requisite usage rate.
The use of a powered heat exchanger to vaporize the liquefied gas offers the advantage of much reduced size, to achieve any given vaporization and gas flow rate, as compared to an atmospheric vaporizer. However, the use of a powered heat exchanger requires significant expenditure for energy. Furthermore the reliability of a powered heat exchanger is highly dependent on the reliability of the power supply. Should power be cut off, the powered heat exchanger will very rapidly reach a state wherein it is unable to supply product gas to the use point at the requisite usage rate.
Whether an atmospheric vaporizer or a powered heat exchanger is used to vaporize the liquefied gas is an engineering decision based, inter alia, on the space availability, equipment costs, and the power costs at a particular usage site.
Whichever type of heat exchanger is employed it must be sized properly so that its rated capacity is at least equal to the required or design gas usage rate at the usage site. While the rated capacity may be somewhat in excess of the gas usage rate, it is not desirable that the rated capacity significantly exceed the gas usage rate because of the incurrence of an unnecessarily high initial capital cost. Furthermore, design overcapacity does not solve the reliability problems discussed above. An overly large powered heat exchanger will become inoperable essentially just as quickly as a powered heat exchanger or the correct size once the power is cut off, and an atmospheric vaporizer will foul due to frost and ice buildup essentially just as erratically whether it is sized correctly or has a rated capacity much larger than needed.
One system that has been used to heat liquefied gases comprises an atmospheric vaporizer to raise the product temperature from cryogenic, e.g., about -300.degree. F., to near ambient temperature, i.e., about -50.degree. F., and then a small heater to raise the product temperature to close to ambient temperature of about 50.degree. F. This system, although more complicated than a system using a single type of heat exchanger, may be more efficient in that it takes advantage of the large temperature gradient between atmospheric and cryogenic conditions to raise the product temperature to near ambient conditions and then, as the temperature gradient shrinks and the rate of heat exchange decreases for an atmospheric unit, the powered heater boosts the heat exchange rate to get the product gas to ambient temperature. Typically in such a system the atmospheric vaporizer has a rated capacity of about 80 percent of the required heat duty and the heater has a rated capacity of about 20 percent of the required heat duty. While this system may offer certain advantages in efficiency it does not address the reliability problems discussed above.
Liquid storage and vaporization systems are intended to operate for long periods without direct monitoring by personnel. Furthermore, these systems are often used as gas backup supply for critical applications where loss of product gas would cause significant material in progress losses or expose equipment or personnel to damage or danger. Thus the issue of reliability is very important and it is very desirable to have a system which exhibits greater reliability than do presently available systems. One way to increase reliability is to have a complete backup system ready to operate once the primary system fails. However, this method may not adequately address the reliability problems. For example, a power loss may affect the backup and associated controls just as it affects the primary unit.
Accordingly, it is an object of this invention to provide a method for more reliably supplying gas from liquid in a liquid reservoir at a given usage rate.
It is another object of this invention to provide an apparatus for more reliably supplying gas from liquid in a liquid reservoir at a given usage rate.